Can the Carbonated Layer Protect Wellbore Cement During Geologic CO2 Sequestration?

Abstract:

Understanding and improving the integrity of wellbores are crucial to prevent CO₂ leakage during geologic CO₂ sequestration (GCS). With advanced knowledge, cement deterioration caused by injected CO₂ can be minimized. We have experimentally analyzed the chemical and mechanical property changes of Portland cement paste samples after 10 days of exposure to 0.5 M NaCl brine saturated with 100 bar CO₂ at 95 ^oC. After exposure, the 3 mm thick cement samples had a total CO₂-attacked depth of 1220 μm from both sides, including a 960 μm thick portlandite-depleted region next to the intact core, a 100 μm thick carbonated layer, and a 170 μm surface layer. The portlandite-depleted zone developed abundant micro-cracks and showed a decreased hardness. A hard carbonated layer which developed near the sample surface could not protect the cement due to formation of this portlandite-depleted zone, where abundant micro-cracks accounted for a 90% decrease in strength of the bulk sample.

Using the reactive transport code CrunchTope, we further investigated the mechanism of portlandite-depleted zone formation. The cement deterioration process was simulated with a 1-D continuum model that captured the dissolution of the portlandite and the formation of a calcite zone closer to the sample edge. Modeling results highlighted that the apparent bypass of CO₂ through the carbonated layer is critical for the evolution of the portlandite-depleted zone, since otherwise the 1-D model predicts complete clogging of the porosity. Defects within the carbonated zone could be due to reaction-induced fractures or to the heterogeneity of the cement. We also incorporated nucleation kinetics for secondary calcite precipitation using previously obtained thermodynamic parameters. We found that the nucleation energy barrier does not suppress calcite formation and thus cannot explain the absence of calcite in the portlandite-depleted zone. The findings from our study help further our understanding of CO₂ attack on wellbore cement during GCS, and improve our ability to predict cement seal integrity.